Testing of integrated circuits (IC's), also called chips or dies, is an important part of the design and manufacture of the circuits. Initial testing is usually performed while the chips are still held together as parts of a semiconductor slice or wafer. Such testing requires a microprobe device that contacts the test pads on the chip and provides connections to the instruments employed for testing the circuits. Standard testing instruments for such circuits operate at a 50 ohm impedance level. Most integrated circuits operate at impedance levels that differ from 50 ohms. The impedance mismatch between the integrated circuit and the testing instruments makes accurate testing difficult and in some cases impossible. Fortunately methods for impedance matching are very well known in the electrical engineering art. There are reactive networks described for example on pages 206 to 215 of The Radio Engineer's Handbook by Terman, McGraw-Hill, 1943 (Reference 1). Another reference is Radio Engineering, third edition by Terman, McGraw-Hill, 1947 (Reference 2) where pages 100 to 104 describe the same reactive networks referred to above and pages 104 to 109 describe transmission line methods including tapered transmission lines, quarter wave matching sections and shorted stubs. A third reference is The ARRL Antenna Book, Published by The American Radio Relay League, Newington, Conn., 1994. The use of transformers to match impedances is also well known.
Another testing difficulty occurs when the chip to be tested has balanced input or output circuits. This presents a problem because the testing instruments are almost universally unbalanced with system ground on the external shield of 50 ohm coaxial cables and with the test signals on the center conductors. Fortunately circuits to convert from balanced to unbalanced modes (commonly referred to as Baluns) are also well known. See for example page 690 of the reference 2 above, and pages 26-9 to 26-13 of Ref. 3. Although application of the techniques of impedance matching are well known, particular structures for implementing those techniques in ways that are advantageous in the microprobing environment may not be obvious.
This specification describes an impedance matching probe employing a tapered transmission line that includes mechanically resilient contacting tips and structures to allow impedance matching over a broad frequency range and large impedance ratios.